A common automotive radio antenna (hereinafter “antenna”) comprises a conductive rod attached to the body of a vehicle. The body of the vehicle is generally conductive and serves as a ground plane for the antenna. The antenna receives signals from all directions substantially perpendicular to the axis of the antenna (i.e., primarily horizontally) in patterns referred to as lobes. The three dimensional space within which the signals are received or detected is referred to hereinafter as the capture area. A conductive rod antenna is a resonant antenna, which implies that a relationship exists between the wavelength of the desired signal reception and the length of the antenna. Generally, utilizing an antenna length of one quarter of the wavelength of the desired signal maximizes the reception of a resonant antenna within a single lobe. Therefore, resonant antennas are practical for short wavelengths, or high frequencies such as FM radio frequencies. For example, the FM frequency wavelengths range from approximately two to four meters, and the corresponding antenna length is usually approximately seventy-five centimeters. As wavelengths become longer, resonant antennas become less practical.
Frequencies below approximately 10 Mhz such as AM radio frequencies, have wavelengths that are too long to detect with a resonant antenna in an automobile application. A contemporary form factor for reception of AM signals in an automobile is a plate antenna. In contrast to the conductive rod antenna, a plate antenna is a non-resonant antenna that operates by detecting the voltage field between two plates. In automotive applications, the plate antenna includes a first metallic plate coupled to a metallic portion of the automotive frame. The capture area, or voltage field detection area, is substantially between the first metallic plate and the metallic portion of the automotive frame. Plate antennas are commonly integrated into the automotive structure (i.e., no portion extends outward from the vehicle). The location and placement of the first plate in a plate antenna is generally dictated by the size of the first plate.
In plate antenna design, the relationship between the capture area for voltage field detection and plate antenna size is approximately linear; therefore, maximizing plate antenna size is favored in design. Consequently, what is lacking is a method for increasing the plate antenna capture area without increasing the plate antenna size.
Accordingly, a method for increasing the plate antenna capture area without increasing the plate antenna size is desirable. Such a method would enable an antenna system that reduces the overall weight of the vehicle and increases the options for positioning of the antenna within the vehicle.
Other desirable features will become apparent from the following detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.